The invention relates to a brake booster for a motor vehicle.
Boosted brakes such as disclosed in U.S. Pat. Nos. 6,494,125 and 7,100,997 for use in a motor vehicle commonly comprise a pneumatic brake booster for actuating a master cylinder. The brake booster usually has a rigid casing in which a transverse partition that sealably delimiting a front chamber subjected to a first pressure from a rear chamber subjected to a second pressure varying between the first pressure and a pressure greater than the first pressure causing corresponding movement. The brake booster has a moving piston fixed to the moving partition that has a front face which acts on a primary piston of the master cylinder by way of a reaction disc retained in a cage. The cage is located between the moving piston and the primary piston and is connected to a control rod which selectively moves in the piston as a function of an axial input force exerted against a return force acting on the control rod by a return spring. A first plunger arranged in front of the control rod in the piston has a rear end with at least one annular rear seat for a three-way valve. The three-way valve progressively moves between a first position in which, with the control rod at rest, the front chamber and the rear chamber are in communication, and a second position in which, with the control rod actuated, the second pressure prevailing in the rear chamber increases. The three-way valve places the rear chamber in communication with the pressure which is greater than the first pressure to effect a brake application. A feeler defined by a second plunger is located on the front end of the first plunger passes through a bore leading from the piston and the control rod and in the rest position, the second plunger is arranged at a defined distance from the reaction disc. When the control rod is actuated with an input force whose intensity is greater than a first defined intensity moves the feeler or second plunger into contact with the reaction disc and as a result a reaction force from the master cylinder is transmitted into the first plunger and the control rod. The ratio of the area of the reaction disc in contact with the cage to the area of the feeler or second plunger in contact with the reaction disc defines a first defined boost ratio. The cage comprises at least one moving decompression wall which, when the control rod is actuated with an input force whose intensity is greater than a second defined intensity greater than the first moves so as to create in the cage an additional volume in which a front part of the reaction disc expands to reduce the reaction force transmitted to the feeler or second plunger by way of the rear face of the reaction disc. The ratio of the area of the reaction disc in contact with the cage to the area of the feeler or second plunger in contact with the reaction disc defining a second boost ratio, which is greater than the first boost ratio.
In the boosted brake disclosed in U.S. Pat. No. 7,100,997, the moving decompression wall forms part the cage that is located between the reaction disc and the primary piston of the master cylinder. The cage has a housing with a first face attached to the reaction disc and a passage to retain a cylindrical decompression piston in contact with the reaction disc substantially along the axis of the plunger. The decompression piston is elastically urged toward the reaction disc by a helical spring which is housed inside the housing and the helical spring that has substantially the same diameter as the decompression piston. When the intensity of the input force exceeds the second defined value, the decompression piston is pushed back in the housing, compressing the helical spring to create a free volume that allows the reaction disc to be further decompressed and limit the ratio of the input force in the development of a corresponding output force.
While this type of decompression wall has many advantages it unfortunately has a disadvantage in terms of space requirement. Specifically, this design entails the use of a helical spring that has substantially a same diameter as the decompression piston. In is impossible to reduce the diameter of the helical spring without reducing the diameter of the decompression piston and as a result an appropriate surface area for creating in the cage a decompression volume would not be sufficient to bring about suitable decompression of the reaction disc. Further, a reduction in the size of the helical spring may not create a force that is capable to move the push rod located between the cage and the primary piston of the master cylinder in a desirable manner.